In document handling systems, documents such as checks, currency, letters and the like, are transported one by one successively to the inserter system with a relatively high velocity up to 150 inches per second. The document detection system of the inserter system is intended to recognize information from each document and to supply this information to a processing means.
However, an undesirable situation may occur when two or more documents are fed to the inserter system at the same time. This situation, known as a double-feed document condition, must be detected immediately in order to generate an alarm signal and/or to shut down the document handling system. Presently, these document detection systems are either mechanical or electro-optical.
The known mechanical technique (described, for example, in U.S. Pat. No. 3,679,202) involves setting the gap of a roller under which the paper travels. When a paper having a thickness greater than the desired paper thickness passes through the roller (as would be the case for a double-feed document condition) an alarm signal is produced. This known technique is very difficult to set for thin documents, and the system frequently runs out of adjustment because of vibration, wear and the like.
Electro-optical double detection systems (as described in U.S. Pat. Nos. 3,873,843 and 4,906,842) take advantage of a light emitter and a corresponding light detector, both positioned on opposite sides of the feed path for the documents. The light emitter generates a beam of light which passes through the document(s) in the inserter system and is detected by the light detector. The light detector produces an output signal which is a function of the opacity of the document(s) between the light emitter and the light detector. The output signal is calibrated to a normal condition in which a single document is fed to the inserter system. However, when a double-feed document condition occurs, the output signal changes sufficiently so as to cause an alarm signal.
In a double detection system presently used by the assignee of the present invention, two sets of light emitting diodes (LEDs) and detectors are employed, and the setting of five interrelated potentiometers is required.
In these electro-optical double detection systems, the output level setup procedure is tedious and time consuming. Moreover, variations and deterioration of circuitry components, voltage variation over time, slight misalignment, different color of paper, presence of paper dust and the like, require continual adjustment of the preset signals of the light emitter and light detector.
Additionally, U.S. Pat. No. 5,067,704 describes an apparatus which includes a light control circuit for controlling the intensity of the light emitter (by switching in various resistors), a variable gain amplifier for amplifying the output of the light detector, and a central processing unit (CPU) for setting the intensity of the light emitter and the gain of the amplifier. The output of the amplifier determines a double-feed document condition. This apparatus, however, requires an operator involvement.
Another disadvantage of these electro-optical double detection systems is that the detector output signal can increase due to a black mark or a logo on the document, paper flutter, etc.
Therefore, it would be desirable to provide a simple and reliable method of calibration without substantial and time-consuming operator involvement to distinguish between false and true double document conditions, and to detect a slight overlapping or slight gap of adjacent documents.